The present invention relates to an optical transmission system and method using wavelength division multiplex.
There is an optical system wherein information is digitized and optically modulated on a transmission side and transmitted through an optical fiber serving as a transmission path, and an optical signal transmitted through the optical fiber is converted to an electric signal and restored to the digital signal on a reception side, thus reproducing the original information. This system is one of communication systems capable of providing a wide band, achieving high-speed transmission, and transmitting large-volume information with high quality. Such an optical transmission system has been researched and developed more and more with development of optical fiber amplifiers, and special attention has been paid to the system as a large-capacity transmission system for a future multimedia information age.
In optical communications, and in an optical fiber has a very wide frequency band, thereby a great number of communication channels can been provided by multiplexing optical signals with different carrier frequencies. This system is called wavelength division multiplex optical transmission. According to the wavelength division multiplex optical transmission, the transmission capacity of the entire system can be increased without increasing the transmission capacity for each channel, by multiplexing a plurality of channels in wavelength regions. In addition, the wide-band and large-capacity of optical fibers can be efficiently utilized in a wavelength division multiplex optical transmission system wherein a node having a function of drop-multiplexing a channel in a transmission path and a function of add-multiplexing a channel in the transmission path is provided in the transmission path.
An ADM node, which is a kind of the above-mentioned node, comprises a demultiplexer for demultiplexing a plurality of channels to a channel in accordance with a given wavelength, a multiplexer for multiplexing, a switch for drop-multiplexing a desire channel in the demultiplexed channels and add-multiplexing a desire channel to the multiplexed channels. As typical example, the multiplexer and demultiplexer include, respectively, input ports or output ports for at least several channels, thereby to multiplex/demultiplex optical signals which are associated with different channels and are obtained by modulating carrier optical signals having different wavelengths for the respective channels. These ports have wavelength dependency and have wavelength/transmittance characteristics as shown in FIG. 2. Specifically, each port has such wavelength/transmittance characteristics that the port transmits only wavelengths in a predetermined range with respect to a central frequency of the wavelength of a channel associated with the port. In addition, the wavelength/transmittance characteristics vary slightly from multiplexer/demultiplexer to multiplexer/demultiplexer. If the number of ADM nodes to be inserted in the transmission path increases, multiplexers/demultiplexers having different inherent wavelength/transmittance characteristics are connected and thus band-limit regions in all channels will increase and transmission bands will narrow. For example, if an M-number of ADM nodes are provided in the transmission path, it is considered that there are an optical transmission terminal multiplexer, a demultiplexer and a multiplexer in each ADM node and an optical reception terminal demultiplexer. In total, an "2M+2" multiplexers/demultiplexers having wavelength/transmittance characteristics are connected. In a case where wavelength/transmittance characteristics of each port of the multiplexer/demultiplexer are of a Gauss type with a half-maximum transmission band width Bf, the half-maximum transmission band width becomes Bf/(2M+2).sup.1/2 after passage of signals through the "2M+2" multiplexers/demultiplexers. Thus, the transmission band width greatly decreases in accordance with the number of ADM nodes. This poses a serious problem in an optical transmission system using wavelength division multiplex and NRZ (Non-Return-to-Zero) pulses. Specifically, when digitized data is to be transmitted in the form of optical signals, the digital data is converted to pulse signals and, with use of the pulse signals, a continuous carrier light of a predetermined wavelength is modulated. In this case, NRZ pulses are conventionally used as the pulse signals. The reason is that NRZ pulses have a less increase in optical spectrum band and are suitable for high-density wavelength division multiplex.
However, when an NRZ-pulse optical signal passes through a device having a narrow-band wavelength/transmittance characteristics, if the carrier wavelength of the optical signal has a misalignment in wavelength relative to the transmission central wavelength of the device, the optical spectrum becomes asymmetric and a large optical waveform distortion occurs.
Under the circumstances, in an optical transmission system using wavelength division multiplex and NRZ-pulse optical signals and also having ADM nodes in a transmission passage, the transmission band is narrowed by the multiplexer/demultiplexer. Consequently, a large optical waveform distortion occurs due to a misalignment between the wavelength of a light source and the transmission central wavelength of the multiplexer/demultiplexer.
As has been described above, in the conventional optical transmission system using wavelength division multiplex and NRZ-pulse optical signals suitable for high-density wavelength division multiplex, if ADM nodes are provided in the transmission path, the optical spectrum of the transmitted optical signal becomes asymmetric due to the misalignment between the wavelength of the light source on the optical transmitter side and the transmission central wavelength in the multiplexer/demultiplexer.
It is considered that the optical waveform distortion increases due to such an asymmetric spectrum. In order to maintain high communication quality and prevent failure of communication, it is necessary, therefore, to make the wavelength of the light source coincide with the transmission central wavelength of each multiplexer/demultiplexer. In order words, strict conditions must be satisfied for stability of wavelength/transmittance characteristics of the light source and multiplexer/demultiplexer.
The conditions for stability of wavelength become stricter as the number of ADM nodes inserted in the transmission path increases. Thus, the number of ADM nodes to be inserted is limited in accordance with the degree of wavelength stability required in the communication system. However, it is technically difficult to make the wavelength of the light source for all channels coincide stably with the transmission central wavelength of the multiplexer/demultiplexer and to maintain the state of coincidence, in consideration of the wavelength of the light source, a fluctuation in wavelength transmission characteristics of the multiplexer/demultiplexer due to ambient temperatures, degradation with passing of time of the light source and multiplexer/demultiplexer, etc.
Taking into account the development of information-oriented societies as well as development and wider use of information-related devices, there will be an increasing demand for communication systems having a great number of ADM nodes in a transmission path, which ADM nodes have functions of adding (or branching) and dropping (or inserting) channels. Under the circumstances, there is a demand for development of techniques which will make it possible to increase the number of ADM nodes, with less degradation due to temperatures or passage of time. Besides, even in a case where there is no ADM node in the transmission path, the optical waveform deteriorates if a wavelength misalignment occurs between the wavelength of the light source and the transmission central wavelength of an optical filter with a narrow transmission band, which is provided in the transmission path, or on the optical transmission terminal side or optical reception terminal side. In particular, in a demultiplexer which is widely used for high-density wavelength division multiplex, a plurality of channels are densely arranged in a wavelength region. Thus, the transmission band for each channel is not sufficiently broader than the band of the optical signal. Moreover, it is very difficult from a technical aspect to make the wavelength of the light source coincide with the transmission central wavelength of the optical filter including the multiplexer/demultiplexer for all channels.
A first object of the present invention is to provide an optical transmission system using wavelength division multiplex, which can reduce an optical waveform distortion due to a wavelength misalignment between the wavelength of a light source and the transmission central wavelength of a multiplexer/demultiplexer, relax the conditions for stability of wavelength transmission characteristics for the light source and multiplexer/demultiplexer, and increase the number of ADM nodes which can be inserted in the transmission path, thereby enhancing reliability and stability.
A second object of the present invention is to provide an optical transmission system using wavelength division multiplex, which can reduce an optical waveform distortion due to a wavelength misalignment between the wavelength of a light source and the transmission central wavelength of an optical filter in a transmission path, and relax the conditions for stability of wavelength transmission characteristics for the light source and optical filter, thereby enhancing reliability and stability.